This invention relates to facsimile units including transmitters, receivers or transceivers. More particularly, this invention relates to a system where a document is scanned in a facsimile transmitter or transceiver to generate electrical information-bearing signals representing dark-light variations in the document being scanned. These information-bearing signals are transmitted over a communications network to a facsimile receiver or transceiver where the information-bearing signals are converted to marks or images on the copy medium so as to form a copy which is a facsimile of the original document.
In my copending application Ser. No. 622,215, filed Oct. 14, 1975, I disclosed a facsimile transceiver comprising a rotatably mounted drum and an information transducer juxtaposed to the drum for marking on a copy medium carried by the drum or detecting dark-light variations in a document carried by the drum. The drum is driven by a DC motor with the speed of the motor controlled by a phase comparison of tachometer pulses and reference pulses.
In copending application Ser. No. 622,214, filed Oct. 14, 1975, and assigned to the assignee of this invention, a DC motor is also utilized to drive a rotatable drum of a facsimile transceiver. The square wave output voltage responsive to the comparison between the tachometer pulses and the reference pulses is applied to the DC motor where the square wave has a duty cycle of approximately 50% when the frequency of the reference pulses corresponds to the frequency of the tachometer pulses. Thus, as the duty cycle of the square wave increases (or decreases) the duration of the drive pulses in the square wave increases (or decreases) and the duration of coasting between drive pulses decreases (or increases). In this manner, the speed of the DC motor is controlled to achieve reference pulses and tachometer pulses of substantially the same frequency.
In the facsimile motor drive circuits described in the aforesaid applications, the period of coasting between motor drive pulses represents a period in which the motor is essentially out of control. This out-of-control condition characteristically produces a jitter or oscillation in the drum of the facsimile transceiver as the drum seeks to maintain a desired frequency or speed, i.e., the drum fails to maintain the desired speed but rather jitters or oscillates about the desired speed. Such jitter or oscillation adversely affects the facsimile. The problem becomes particularly severe where the inertia of the drum is high and friction is low and the drum and motor are slow to decelerate to the desired speeds. The problem is worst at relatively high scanning speeds.